Zs measuring - ring

[JuniorSparky]

Hi all,

I know it's easier just to add R1 + R2 to Ze to get Zs, but I was wondering if I want to measure Zs on a ring circuit which one will be considered the furthest point - the center of the ring or the end of the circuit where one leg's disconnected at the board?

 

[Richard Burns]

The highest resistance will be found at the centre point of the ring and this would be the highest Zs, assuming that there are no spurs off anywhere.

 

[telectrix]

and why on earth would you consider measuring Zs of a RFC with 1 leg dissed? it would not be a RFC then, it's be a radial.

 

[JuniorSparky]

Hahaha sometimes I have ridiculous ideas I like to try although they make no sense

Thanks for the answers

 

[trev]

You might also want to have a read of GN3 mate, you should test at every point of utilisation (including spurs) and record the highest value.
In my world, I measure everything. I never calculate values for test sheets.

 

[JuniorSparky]

Hi trev,
i was just wondering how would I know where exactly the centre if I don't take a reading from all of them. It's obvious to take a reading from all, but it didn't cross my mind. When reading (not this forum particularly) around I was left with the feeling most people prefer to calculate Zs. That's why I was wondering about getting the reading through testing!

 

[Richard Burns]

If you are installing a new circuit you might be able to work out where the exact centre of the ring is, but there is no guarantee that you would have a socket there to measure from!
If you have an existing circuit it is not generally possible to tell the cable runs and find the centre.

Doing the figure 8 interconnection test along with the end to end resistance provides confirmation of ring status, continuity of cpc, polarity and the maximum possible R1+R2 value for the ring.

 

[trev]

Hi trev,
i was just wondering how would I know where exactly the centre if I don't take a reading from all of them. It's obvious to take a reading from all, but it didn't cross my mind. When reading (not this forum particularly) around I was left with the feeling most people prefer to calculate Zs. That's why I was wondering about getting the reading through testing!

The reason why you take the measurements from every point is that you're proving polarity at every point, then you're taking the Zs at every point including spurs.
On an existing install you'd be guessing where the mid point is, another reason to test at every point, but if you take the live end to end measurement (R1) and add it to the CPC end to end measurement (R2) and add the 2 then divide the total by 4 you get a rough guide to what to expect.

 

[JuniorSparky]

Thanks guys. All clear I had that as an extreme case for example the nail in the middle as Richard has suggested. As trev said I can calculated and get a roughly idea what to expect. Then I can compare it with the readings from each socket, see which one is higher between the measured and the calculated values and use that value as the highest Zs

 

[pennychew]

If you are installing a new circuit you might be able to work out where the exact centre of the ring is, but there is no guarantee that you would have a socket there to measure from!
If you have an existing circuit it is not generally possible to tell the cable runs and find the centre.

Doing the figure 8 interconnection test along with the end to end resistance provides confirmation of ring status, continuity of cpc, polarity and the maximum possible R1+R2 value for the ring.
View attachment 28781
View attachment 28782

Ive just printed those out for a look at later, better than the explanation in GN3!

 

[Paul.M]

And I've saved them to post up next time someone asks about RFC Zs testing

 

[Richard Burns]

The calculation is a slight simplification to make it easier to understand, but the numbers calculate the same for the ring midpoint.
The numbers get a bit more complex once you move away from the ring centre, I get values for the figure 8 testing ranging from 0.24 to 0.25 times r1+r2, the lower numbers being near the CU, but I could have got my calculations mixed up. Trying to remember how to expand and contract quadratic equations and solve is proving more than my soggy brain can handle.:dunce2:.

 

[happysteve]

The calculation is a slight simplification to make it easier to understand, but the numbers calculate the same for the ring midpoint.
The numbers get a bit more complex once you move away from the ring centre, I get values for the figure 8 testing ranging from 0.24 to 0.25 times r1+r2, the lower numbers being near the CU, but I could have got my calculations mixed up. Trying to remember how to expand and contract quadratic equations and solve is proving more than my soggy brain can handle.:dunce2:.

I just get the computer to do it. Prints a nice pretty graph, too.

I've posted this before. Here's a graph showing you the variations in (r1+r2)/4 for different ratios of line conductor and CPC.



If EITHER the CPC and line conductors are the same CSA, OR you're exactly half way around the RFC, then R1+R2 will be equal to (r1+r2) / 4. If the CSA of the CPC is different from the CSA of the line conductor, then R1+R2 will be a bit less than (r1+r2) / 4.

In the real world, you're more likely to get a discrepancy due to uncertainties in the measurement, and other factors such as extra resistance in joints (all those terminations in sockets) etc.

 

[Richard Burns]

I think you are doing a slightly different calculation there, You are demonstrating that (r1+r2)/4 <> R1+R2 if you consider all ring points.
This is because the maximum R1+R2 is only at the midpoint of the ring and it decreases as you get closer to the origin (with differing csas).

When measuring (r1+r2)/4 in the figure 8 every result you measure is the maximum resistance, which is not the same as R1+R2 at each point on the ring, it is only the same as the maximum R1+R2 on the ring (which of course is the worst case scenario you are trying to determine).

I was trying to demonstrate that (r1+r2)/4 measurement is not quite precise at each point in the ring.
when I got to this equation
16*((r1*r2)+7r22+7r12+49r1+49r2)/ (r1*r2)+7r22+7r12+49r1+49r2
I gave up.

 

[telectrix]

sometimes we can get blinded by science. back to to tired asnd trusted methods of years ago......switch on, it works, it don't go BANG. sorted.